Fluorescent retouching materials



July 10, 1956 B. BERRY 2,754,427

FLUORESCENT RETOUCHING MATERIALS Filed Aug. 15, 1952 PAINT PHOTOGRAPHIC EMULSION IN V EN TOR.

gulifl. $6

A TTO/P/VE VS United States Patent FLUORESCENT RETOUCHING MATERIALS Burtt L. Berry, San Francisco, Calif.

Application August 15, 1952, Serial No. 304,617

7 Claims. (Cl. 250-71) This invention relates to copy intended for reproduction by photomechanical methods and to materials for producing such copy.

lrior Patent No. 2,395,986, issued March 5, 1946, to the present inventor and entitled Half-Tone Drop-Out Process, discloses a method for highlighting photomechanical reproductions of photographic copy by making continuous-tone photographic prints, intended for such reproduction, upon a photographic paper, the substratum or background whereof is luminescent when radiated by ultraviolet light, or by light including both ultraviolet radiation and radiation in the short wavelength portion of the visible spectrum. When such copy is used, the ordinary half-tone exposure is made, and, in addition thereto, a booster is made with light emitted from the background under an exciting illumination of the type mentioned. The purpose of such highlighting is to increase the range of contrast in the photomechanically reproduced picture. Pictures printed from half-tone plates so produced have a crispness and brilliancy greatly exceeding those made without highlighting treatment.

The fact that contrast is enhanced by the use of luminescent background materials is due primarily to two facts; first, that no photographic emulsion has an unlimited contrast range, and second, that the blackest portions of photographic copy do reflect light to some extent. The result is that in making plates for a photomechanical reproduction from photographic copy there is ordinarily a degradation of contrast at each step of the process. If the negative is to be so fully exposed as to give maximum density in the highlight portions there will usually be suflicient reflection from the portions which are intended to be totally black to darken the negative in some degree. If, however, the exposure is made by the lightemitted by the copy itself, i. e., by its own luminescence, there need be no exposure whatsoever from the black and the full range of the photographic emulsion can be employed. Although a continuous-tone photograph on a luminescent background possesses inherently the possibility of accomplishing this heightening of contrast, there are many situations where it is not practical with methods and materials heretofore available. The primary reason why this has been so lies in the fact that continuous-tone photographs which are to be used as copy for photomechanical reproduction usually require at least some degree of retouching. The half-tone or other reproduction process used may distort certain of the tone values. Flaws in either the negative or in the positive print produced therefrom may need to be covered or corrected, and illustrations for advertising purposes may require that some particular feature be emphasized or subdued.

Retouching of this character is ordinarily done with gray water-color paints and those practicing it acquire a high degree of skill in blending the tones of the paint with those of the photographic image on the copy. The materials used for such retouching are usually opaque water colors, generally similar to tempera, which do not set permanently, as with oil paints, but can be washed off so that the work can be started anew in case of errors in the retouching procedure. In order to facilitate the blending of the tones retouchers frequently use a gray scale which is a half-tone print including a number of blocks grading from black to white in successive stages, differing successively by equal increments of exposed background. At the white end of the scale none of the background is covered by the black, half-tone dots. Successive steps in the scale have the background covered by ten percent, twenty percent, thirty percent, etc., up to full black, where one hundred percent of the background is covered by the tone. Alternatively, such a gray scale may be matched by a similar succession of blocks in which the degree of reflectance (which is approximately proportional to the amount of background exposed) is achieved in continuous tone by varying the photographic exposure.

Gray pigments are ordinarily secured by mixing black and white pigments. Mixing an equal amount of a black pigment, such as bone black or carbon black, and a white pigment, such as white lead (basic lead carbonate), zinc white or titanium oxide, will not, usually, produce a fifty percent tone on the gray scale. The pigments difler as between themselves in covering power. Thus, a small amount of titanium oxide will lighten the tone of the resulting paint to a much greater degree than will an equal amount of white lead, andthe various blacks also differ between themselves in their ability to darken a mixture. This is true even as between the various carbon blacks; bone black, prepared from animal charcoal, will darken a paint to a greater extent than will lampblack, which is soot from burning oil.

In order to facilitate the matching of a desired tone, therefore, a retoucher usually provides a set of grayscale paints corresponding, respectively, to the various steps of his photographic gray scale. With a set of paints so graded the artist doing the retouching can quickly match any desired tone, for although the relationship between the black and white pigments is not linear with respect to the scale as a whole, it is nearly enough so for any small portion of the scale that a mixture of equal part of grays which correspond respectively, say, to seventy percent and eighty percent black will be very closely approximate to a seventy-five percent black.

The method of using the gray scale with corresponding paints is fairly obvious. A retoucher selects, from the scale itself, the tone or tones which correspond most nearly to the tone which he wishes to have produced on the final copy. If one of the blocks happens exactly to match the tone he wishes to produce he uses the corresponding paint unmixed, but if the desired tone falls between two adjacent tones of his scale he mixes the two corresponding paints in the proper estimated proportions and then applies them to the copy.

Where the copy to be reproduced is photographed on a fluorescent background or substratum, however, this method of matching tones is not successful. As has been pointed out, the over-all contrast in the final reproduction is greatly enhanced where the fluorescent background is used in making reproductions from photographic copy. When the process is skillfully carried out full highlighting, or the suppression of all half-tone dots in the highlight area, can be accomplished, and the increase in light reflected from such areas, over that reflected from areas carrying the minimum size of dot which can be produced with ordinary photographic copy, may be as much as thirty percent. There will, of course, be no change in reflectivity from the black areas, where the half-tone dots in the final positive are fully closed in.

What is desired is that the intermediate steps in the scale should still remain proportional. If the normal gray scale is used with its corresponding paints, the thirty percent additional exposure of the half-tone negative, resulting fromthe fluorescence of the background inthe intermediate tones, will not be present in the retouched portion. Worse, although a fifty percent white would appear to match by eye when the retouched copy is viewed by normal light, when illuminated by ultraviolet in making the booster exposure no further exposure of the film would be produced from the retouched portions. Hence, assuming that the whites are increased by a desired thirty percent, a fifty percent white, which should be increased by an additional fifteen percent, would not be so modified, but would appear in the final half-tone cut as approximately a thirty-five percent white, much darker than intended. With much training and experience a retoucher may learn to make the necessary compensation with respect to the darker end of the gray scale; with respect to tones approaching the highlights such compensation is impossible. In the past, therefore, the use of fluorescent copy in this mannerhas been limited either to mere highlighting or to unretouched photographic copy.

The primary object of this invention is to provide a type of retouched copy in which the reflective values of all the tones above black in the final reproduction are uniformly increased. To this end, other objects of the invention are to provide a retouching paint which will match the intermediate tones of photographs made on fluorescent or luminescent backgrounds, both as used by eye under white light and as used by the process camera when illuminated by ultraviolet light or by mixed ultraviolet and violet light; to provide a set of such paints which will correspond to a uniformly spaced gray scale under either condition of illumination; and to provide such paints and sets of paints which will maintain their ratios of reflectance and emissivity indefinitely. V

In accordance with the present invention the final product is photographic copy for photomechanical reproduction comprising a photographic print formed on a luminescent background and including retouched' portions having gray tones overlying the photographic image and comprising a white pigment, a black pigment, and a luminescent material, held by suitable binderand so compounded that the gray tones of the unretouchedportion "of the print and of the retouched portions whichmatch when viewed by reflected light, also produce. equalexposures when photographed by their ownluminescence. The preparation of such copy involves a number ofelements:

(1') A photographic paper or medium comprising a photographic emulsion, preferably of the silverhalide type, superimposed upon a luminescing background, which may be either a substratum on the paper under the emulsion, or a phosphor or dye incorporated in the body of the paper itself;

(2) A white retouching paint matched with the luminescence of the aforementioned paper or equivalent-material compounded to produce equal intensity of luminescence when activated by exciting radiation of like intensity; and (3) A retouching paint or series of paints comprising a luminescent white, as described under element (2) above, plus a black pigment incorporated in a suitable medium or binder.

The invention and thevarious steps involved in-the practice thereof, will be'better understood from the following'description, taken in connection Withthe accompanying drawings wherein:

Fig. 1 is a diagram, in cross-section, of apor-tion ofa piece of photographicicopy prepared in accordance with this invention;

Fig. '2'is'a diagram showing one arrangement of equipment used in measuring relative tone values of retouching paints prepared in accordance with this invention.

It is basic to the practice of this invention that the luminescent photographic paper and the luminescent white which forms the basis for the retouching materials should substantially photographically match. If, as is usually the case, the type of film or plate on which the copy itself is to be prepared is unknown to the retoucher, this requires a visual match as well. Obviously, it is much easier and more practicable to match the retouching materials to the paper than the reverse; a great majority of photographic materials are manufactured by large organizations, manufacturing in quantity and having facilities for standardizing the materials used. The degree and quality of the luminescence produced in a fluorescent material when irradiated by exciting wavelengths depends on both the wavelengths and the nature of the material itself. The latter may be a fluorescent or phosphorescent dye or it may be a solid phosphor. A photographic paper manufacturer will naturally endeavor to obtain the maximum possible degree of luminescence compatible in an otherwise satisfactory paper. Photographic papers intended for photomechanical reproduction should, to produce the best results, be pure white, with highly reflective surfaces. The luminescence produced when excited by a short wavelength radiation (usually below the blue in the spectrum and extending into the ultraviolet) is desired to have the maximum photographic effect. The method of incorporating the luminescence in the material will inevitably have at least some minor effect upon the other properties desired, and diflerent manufacturers will, in general, adopt diflerent compromises as between optimum values of one or the other properties desired in the final product. Even the same manufacturer may adopt different standards for materials intended for use in slightly ditferent processes. For this reason no absolutes can be established in the description of the retouching media, as there will be diflerences in those adapted for the use with different photographic materials. The following description will therefore be limited to the description of aparticular combination, given in such detail as to enable those skilled in the art to practice the invention or-the various steps thereof with any desired photographic paper as a basis.

In the particular example chosen for illustration, the base material 1 is a high grade pure-white paper stock in which there isincorporated, during manufacture, a substantially colorless fluorescent dye sold under the trade name of-Tinopal BVA. Pure white, in this connection, means that the reflectance 'of a surface of the material is substantially the same for all wavelengths Within the visible spectrum The dye has a high affinity for the paper fibers, and in the materialmentioned is added during the beating process and all excess is washed out during the manufacture of the paper. The stock has a smooth, hard surface and its coefficient of reflection is highover 80 percent.

Overlying the surface of the paper is a layer 7 of photographic emulsion, comprising a dispersion of silver halide in gelatin, of the same type that is normally used in papers intended for copying. It is to be noticed that the particular structure of the photographic material used is not an element of this invention and that the description given above is merely one example of a material on which original copy can be prepared'for using this invention and that any otherpaper cardboard or other medium which permits a photographic image to be superimposed upon a luminescent background may serve as a starting point. The retoucher applies the-paints thereinafter to be described on selected areas of an image developed in the emulsion, as indicated at- 9.

' The material described'fluoresces when irradiated with light in the wavelength-range-between 3,200- and 4,000 Angstroms cmitting a blue-white on light blue luminescence which is highly active photographically on all of the usual emulsions used for the coating. Therefore, even color blind plates or films may be used in photographing copy made on this paper, as will next be described.

The relative emissivities and reflectivities of this paper are next determined. A sample which has not been exposed in any degree (except, perhaps, for some identifying mark) is put through the developing, fixing, and washing processes, in exactly the same manner as though it carried a latent image which it was desired to bring out. When dried in the usual manner this sample forms a norm against which a white pigment is to be standardized.

The white sample of paper 11 is placed on a copy board 13 (Fig. 3) and illuminated from both sides by lamps 15 of the type ordinarily used by photoengravers, e. g., by white flame arcs. Preferably, in making this initial test, the light from these arcs is filtered, the filters 17 used being those which will pass the visible wavelengths of the spectrum but will stop the ultraviolet and violet radiation which would cause fluorescence. Preferably the lights used for the illumination are placed on either side of the copy board so that the light falls upon the sample from each side at an angle of approximately 45. The light reflected from the paper is measured with a suitable light meter or foot-candle meter 19, placed at a known distance directly in front of the paper. The meter is, of course, screened from direct illumination from the lamp. The reading of the meter is then recorded.

A like measurement is then made of the luminescence of the paper itself, substituting, for ordinary white light, black light lamps which are emissive of the short wavelength radiation above mentioned as exciting fluorescence in the paper. In making this second measurement, however, the filter which excludes ultraviolet radiation is placed between the light meter and the reflecting surface, so that it is only fluorescent light which is measured and not any reflected ultraviolet or short wave visible radiation. If, however, the ultraviolet radiation is confined solely to the luminescent surface the filter may be omitted in making this latter measurement, since materials which fluoresce strongly are almost wholly absorptive of the ultraviolet radiation itself. Again a note is made of the foot-candles (or other light unit) as indicated by the meter.

Preferably spectroscopic analysis is made of the composition of the fluorescent emission. In preparing the matching grays for use in this invention it is not neces sary that the spectral composition be identical as long as it falls within the same band of wavelengths. The process plates used in preparing half-tone or continuoustone negatives by most mechanical processes are usually of the color blind type, which are not sensitive to the middle and long-wave components of the visual spectrum but are nearly uniformly sensitive throughout the blue range. Ordinarily, the use of fluorescent materials which luminesce in very nearly the same color, as determined by eye, is accurate enough to give satisfactory results, but materials may exist having unusual spectral characteristics and spectroscopic analysis is safer.

A white paint is then prepared. For the pigment almost any of the well-known whites may be used with the exception of titanium dioxide, which, it has been found, smothers or inhibits fluorescence of substantially all of the luminescent materials. Preferably the pigment chosen is one having high covering power, such as white lead (basic lead carbonate), zinc white (zinc oxide), or the non-fluorescent grades of zinc sulphide. In general, the zinc pi ments are preferred, since they do not darken when exposed to the action of sulphide fumes which may be present where work of this character is done. This material as milled together with a luminescent material is incorporated in a water-soluble binder.

It should be noted here that the white pigments listed above are merely examples of those which may be used, being the most highly reflective and most opaque, and

therefore requiring the minimum proportions in the final product. Any of the other recognized white pigments which may be employed, however, with the exception of titanium white, will cover to a greater or less degree without unduly affecting luminescence. The titanium white, however, will substantially reduce luminescence even when employed in amounts too small to give any substantial degree of opacity. Other white pigments affect the luminescence of the paint largely by dilution and (to some degree) by masking, but the titanium pigment appears to have an actual inhibiting effect on all luminescent materials with which it has been admixed.

Generally it is unnecessary for the compounder of the retouching paints to prepare his own fluorescent materials. Various suppliers list a wide variety of phosphors and dyes, giving their spectral luminescence and the wavelengths to which they will respond. Making the selection from such lists, it has been found that phosphors prepared by the New Jersey Zinc Company and designated by them as phosphors 2210 and 2200, in the proportions of twenty-five percent of the former and seventy-five percent of the latter, give a spectral distribution which is closely comparable to that of the Tinopal dye mentioned as used in the photographic material. This mixture is milled, together with the white pigment, to insure a thorough mixture and suitably fine grinding.

The prepared pigment is incorporated in a watersoluble binder, which may be any of those customarily used for the purpose. One suitable binder is a four percent solution of polyvinyl alcohol in water. Gum arabic may be substituted if desired.

Relative proportions of the pigment and the luminescent material are determined by experiment. In the case of the particular match herein described it was found that, of the dry materials, sixteen parts by weight of the mixed phosphors and two parts by weight of zinc sulphide gave the required results. Ten parts by weight of the four-percent solution of polyvinyl alcohol reduces this to satisfactory working consistency.

Using a mixture of approximately these proportions, a card is prepared of like size to that used in making the measurements on the exposed photographic paper, and a like set of measurements, using both the white light and the black light, are made. The foot-candle reading should correspond quite closely to those obtained from the photographic material. Any deviations which are found will be more likely to occur in the readings of the fluorescent light. With any of the good standard whites the difference in white light reflectivity from that of the standard will at most be only a percent or two. If the foot-candles indicated in the measurement made by fluorescent light are in excess of those recorded in measuring the photographic paper a smaller proportion of the phosphors is used, while if the luminosity is less than that of the paper under these conditions, the proportion of white pigment is decreased. It is to be noted that the phosphors themselves appear to be white, but their covering power is very low while that of such pigments as zinc white or zinc sulphide is relatively great. The pigments are used in such quantities as to give an opaque coating and the test sample is prepared with a coating heavy enough to yield this result. When this is done the photo paper and the pigment sample can match under both conditions. Changes in the proportions of the opaque pigment will make very little difference in the reflectivity of the resulting paint but will make large differences in the fluorescent response.

Under certain circumstances the pigment and the luminescent material may be one and the same, but this will not usually be the case. The grade of zinc sulphide commonly sold as chemically pure is non-luminescent under ultraviolet. Further purification by repeated recrystalization will yield a product which is fluorescent, but it would be only by the rarest chance that the emitted light would match that of the photographic paper. Brilliant luminescence of the zinc sulphide can be obtained by small percentages of various additives, such as salts of manganese, cadmium or silver, but the colors resulting are usually in the longer wavelength end of the spectrum yellows, oranges and reds, depending on the additive used. Ordinarily the photographic paper manufacturer will employ a material yielding a large percentage of its radiation in the blue end of the spectrum, so as to obtain maximum effect with color-blind plates which are not affected by green, yellow or red radiation. However, if the photographic paper to be matched is fluorescent in (say) yellow, the matching paint may be compounded of a zinc sulphide white with an additive which will cause luminescence.

In preparing the fluorescent grays, the basic white paint is mixed with suitable proportions of any of the standard black pigments. By way of illustration a suitable black comprises two parts by weight of dry lampback to five parts of aqueous solution of polyvinyl alcohol, to which is added a small amount of methanol in the proportion of cc. per kilogram of the lampback-polyvinyl alcohol-solution mixture.

The amount of this black to be added to a given amount of the white base in order to secure a desired tone of gray is purely a matter of experiment, due to the differing covering properties of pigments. It would be a very rare circumstance ifit were found that an equal-parts mixture of the black and white resulted in a fifty percent gray tone. in preparing a set of retouching paints the white end of the scale will, of course, be the pure white paint (0% tone) and the black end of the scale (100% tone) be pure black paint. In general, however, intermediate values will contain very much less black than the percentage tone would indicate. This is illustrated by the curve of Fig. 2, which gives the results of admixtures of the black and white paints specified above in terms of the percentages, by weight, of the actual black and white pigments. Even on this basis it will be seen that only about seven and one-half percent of black pigment is required to give a fifty percent tone; this corresponds to less than two percent of the mixed black paint described above. On a volume basis the percentage of black is much greater, but volume measurements are uncertain. The plot with relation to actual pigment content is therefore shown as giving a readable curve of substantial accuracy. The paint blender will ordinarily start with an engravers gray-scale, photographed upon the paper which the retouching paints are intended to match. An initial blend, corresponding to each of the desired gray tones, can be made by eye, matching the tones of the gray scale, these latter being graduated and designed by the relative sizes of the half-tone dots of which it is composed, ten percent of the area being fully exposed (and the remainder unexposed) to produce a ten percent tone, twenty percent of the area exposed to present a twenty percent tone, etc.

With this initial blending, samples are prepared in the same manner as the initial white paint sample and are tested under the same conditions as was the original unexposed photographic paper, illuminated by either white or ultraviolet light. The relative proportions of black and white pigment can then be adjusted so that if, under the conditions of the original measurement, the footcandle meter indicated ten foot-candles, with the successive grays, it indicates successively smaller illuminations as measured under like conditions. If desired, the complete gray scale may be prepared, ranging from ten percent to one hundred percent tone. It has been found satisfactory, however, to use a scale differing in tone by twenty percent steps instead of the ten percent steps a complete scale would require, so that if the reading of the foot-candle meter for white, under the degree of i lumin tion el ted s s an ar we e n tn es, he ay tone N 1 samp e would regi e e gh fee candles, the gray tone No. 2 six foot-candles, gray tone No. 3 four foot-candles, and gray tone No. 4 two footcandles. The gray scale with the tones thus spaced gives sufliciently accurate results and intermediate tones are produced by mixing tones adjacent on the scale in the required proportions.

Very fortunately, it turns out that where the paints are thus prepared, and matched by visual light they will also match by luminescence when excited by the black light used. It follows from this latter fact that the measurements may be made by fluorescent light, and

this may prove, actually, to be somewhat more accurate (although less convenient) than using the white light for measurement purposes. This will be particularly true at the larger tone percentages, since there will be less effect of the reflectivity of the black.

The procedure given above may be shortened somewhat by preparing a few samples with arbitrarily chosen percentages of the black and white paints, measuring their relative reflectivity or emissivity as described above, and plotting percentage light-value against percentage of white (or black) pigment. A smooth curve may be drawn through the points thus plotted, and from this curve may be taken the pigment percentages which will give the desired percentages of reflectance and luminescence.

If the measurements are made by white light the procedure can be further shortened by measuring the reflectance from the surface of the semi-liquid paint itself. This, of course, implies that the copy-board should be horizontal in measuring the standard and the lights directed downward in order to get truly comparable readings. With certain black-lights this latter technique can also be used, but ordinarily it is not recommended because aqueous solutions may be highly absorbent of ultraviolet, even in very thin layers. Therefore, care should be taken to ensure that the emissions fromthe semi-liquid white and a dried surface thereof are the same before reliance can be placed on this last-mentioned procedure.

It has been found that there will be some difference in the percentages used of the black and white pigments when materials, which are nominally the same, are purchased from different suppliers. It has been found, however, that when the materials are always purchased from one source, mixtures compounded with the same proportional quantities of all materials will yield like results. Nevertheless, in manufacture, periodical tests are preferably made in order to insure uniformity.

In making photographic copy with the materials as described, a print is made on the photographic paper in precisely the same manner as would be done using paper having a non-fluorescent background. The print thus made is developed, fixed and dried and is then ready for such retouching operations as may be required. The retoucher then works under ordinary white light, which may be either daylight or artificial light, matching his tones so that the print has the proper desired appearance when viewed by eye. Portions of the resultant copy will be exactly as photographed, but other portions will be changed in tone, blanking out or accentuating certain features of the picture, as the case may be. Were ordinary retouching paints used for this purpose, on a print having a fluorescent background, with the copy illuminated by the'unfiltered light of a white flame arc, the negative resulting would exhibit tones only slightly different from those intended by the retoucher. The white flame arc itself contains components of wavelengths which will excite luminescence, and hence if a non-fluorescent white were used in the white portions the resultant half-tone or continuous tone negative might indicate a slightly gray tone; When a booster exposure is given, however, an addition-density is added to the highlight portions of the neg i e r pres nting the o er d l min nt paper,

red cing th light t an mitted by as. much as thi y P rcent. White retouching would give no added density under the booster exposure. Intermediate gray tones would also be false to a greater or less extent, with a minimum distortion of the tone values occurring at the black end of the scale.

With the set of paints such as is here described the whites from the unretouched portions of the photographic copy will be the same as those from the retouched portions, and the intermediate tone values will vary correspondingly. This result will probably not obtain, of course, if the fluorescence of the photographic background and of the retouching colors are not in the same spectral range; thus, if a color blind process plate be used for making the negative and a phosphor fluorescing in yellow were used in preparing the fluorescent white, the fluorescence would produce little or no effect on the color blind emulsion even though the foot-candles illumination, as registered upon the meter, were the same. If panchromatic plates were used, the use of phosphors having different spectral distribution of luminescence might give satisfactory results. By matching the spectral response, at least approximately, it becomes unnecessary to consider what type of emulsion is to be used in making the negatives from the copy.

As an alternative in preparing the fluorescent grays, dyes may be used to supply the luminescence instead of the phosphors. When this is done it is usually preferable to utilize the same dye that is employed in the background of the photographic paper. This dye may be incorporated in the vehicle used in preparing the paint, or it may be precipitated upon the white pigment, in case it is not soluble in the vehicle. Satisfactory fluorescent grays have been prepared in both of these ways. The use of a dye which is soluble in the vehicle may give unsatisfactory results, however, as a result of the retoucher diluting the vehicle or otherwise changing the proportion of dye to pigment. The unsatisfactory results thus produced are, however, due to unskilled use and not to any basic fault in the process itself. A precipitated dye which is insoluble in the vehicle is practically as stable and constant in proportion as is a phosphor and, properly prepared, is just as satisfactory. Dyes of both types are considered to be within the scope of this invention as are pigments containing additives which make them luminescent.

While it would be possible for the retouching artist himself to compound the fluorescent grays which have been described above, even without making the measurements described but determining proportions entirely by eye, the more practical method of practicing the invention commercially is to manufacture the materials in quantity, and market the various shades of gray paint in sets matched to specific grades and brands of fluorescent photographic papers. The approximate matching of the degree of luminescence of a specific paper and the basic fluorescent white can be achieved photographically, or visually if the two materials are subjected to a luminescence-exciting radiation and viewed simultaneously. The eye can detect very small differences in brilliance in immediately adjacent samples. Controlled manufacture such as is here described is much more certain and accurate.

A complete set of retouching paints as manufactured and graded in accordance with the methods here given becomes a valuable adjunct to the manufacturer of plates for photomechanical reproduction, enabling him to overcome, in large degree, the hitherto inevitable degradation of contrast that normally accompanies every stage in photomechanical reproduction. Reproductions made from such plates greatly exceed in quality and sparkle those made from non-fluorescent copy, giving the range of contrast which is obtainable in highlighted plates but with the contrast more uniformly distributed throughout the tonal scale.

It will be understood that while the invention has been described in connection with photographic copy prepared on paper having a luminescent background, the actual 10 material used for the support is quite immaterial to the invention, and, in fact, forms no part thereof per se, The paper is mentioned merely because it is the most usual material, but the support for the fluorescent layer underlying the emulsion which carries the image may be of cardboard, cloth, plastic or other material. The term luminescent includes both fluorescence and phosphorescence. It makes no difference, except in minor details of the exposure itself, whether the luminescence continues only during the period when it is receiving the exciting radiation or whether the luminescence persists for a greater or less period following such excitation.

It will be clearly evident from the foregoing that many modifications are possible within the teachings given herein and therefore it is not intended to imply, in the ensuing claims, any limitations not specifically expressed therein.

What is claimed is:

l. A gray paint for retouching photographic copy comprising an opaque white pigment other than titanium oxide, a black pigment, and a material luminescent when irradiated by ultraviolet light, and an adhesive.

2. A gray paint in accordance with claim 1 wherein said luminescent material is a phosphor.

3. A set of gray paints for retouching photographic copy each comprising at least one opaque white pigment other than titanium oxide, a material luminescent under ultraviolet radiation, a black pigment and an adhesive, the relative proportions of white pigment and luminescent material in each paint of said set being substantially the same and the proportions of white pigment and black pigment differing as between successive paints of the set.

4. A set of gray paints for retouching photographic copy having fluorescent areas thereon, each paint of said set comprising an admixture of a white paint comprising an opaque pigment, an additive inducing luminescence when irradiated by ultraviolet light and an adhesive binder, and a black pigment, the relative proportions of white paint and black pigment in successive paints of the set being such that the intensity of the light emitted there by when irradiated by ultraviolet light differs from paint to paint by substantially equal percentages.

5. Copy for reproduction by photomechanical processes comprising a background material having a white surface comprising a material luminescent under ultraviolet radiation, a silver photographic image visible in gradations of gray between black and the tone of said background superimposed thereon, and areas of gray paint overlying said photographic image, said gray paint comprising a mixture of a black pigment, an opaque white pigment and a material luminescent under ultraviolet radiation, the relative proportions of white pigment and luminescent material being such that when radiated by ultraviolet light in the absence of an admixture of black pigment, the luminescence produced is substantially equal in intensity to that produced by radiation of the background by ultraviolet light of like wavelength and intensity.

6. Copy for reproduction by photomechanical processes comprising a substantially white base luminescent under ultraviolet radiation, a silver photographic image superimposed thereon through which said base is visible in gradations of gray between black and the white of said base, and areas of opaque gray paint overlying said photographic image, said paint including a component luminescent under ultraviolet radiation in substantially the same hue as the luminescence of said base so proportioned that tones produced by said image which match those of said paint when viewed by visible light also match when said base and said paint are rendered luminescent by ultraviolet radiation.

7. Copy for reproduction by photomechanical processes comprising a paper base incorporating a material luminescent under ultraviolet radiation, a silver photographic image'overlying said base, said image and base forming a'p'rint observable under visible light in gradations of gray, and areas of gray paint overlying said photo'- graphic image, said paint being luminescent under ultraviolet radiation in the same hue as the luminescent material of said base to a degree such that areas of said paint and said print which are observable as of like tones under visible light only are alsoobservable as of like tones when rendered luminescent by ultraviolet radiation.

.' References: Cited in the file of this patent UNITED STATES PATENTS Sherwood Aug. 9, 1881 Hewitt Aug. 17, 1915 Yule June 16, 1942 Yule June 16, 1942 a Murray et al. May 11, 1943 Dobbins May 4, 1948 Stern June 8, 1954 

1. A GRAY PAINT FOR RETOUCHING PHOTOGRAPHIC COPY COMPRISING AN OPAQUE WHITE PIGMENT OTHER THAN TITANIUM OXIDE, A BLACK PIGMENT, AND A MATERIAL LUMINESCENT WHEN IRRADIATED BY ULTRAVIOLET LIGHT, AND AN ADHESIVE. 